Amplifying system



June 19, 1934- B. F. MIESSNER `Er AL AMPLIFYING SYSTEM Filed April 9, -'193:2'

Lot.

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Patented June 19, 1934 'UNITED STATES PATENT ori-ica 1,963,668 .A1/murrine' SYSTEM Benjamin FQ Miessner, Millburn Township,-Essex County, and Charles T. Jacobs, Chatham, N. J.,

This invention relates to amplifying systems employing thermionc vacuum tubes and more specifically to 'the elimination of residual ripple orhumin the output of such amplifiers ener- 5 gized from alternating current. The principles of the invention are useful in connection with such translating into electric oscillations the vibrations of the strings, and -an amplifier and loudspeaker for amplifying and translating into sound such oscillations, as disclosed and claimed broadly in the copending application of Benjaminl F. Miessner, Serial Number 512,399, filed fJan-l uary 30, 1931, on which U. S. Patent 1,929,02

has now been issued.

In such instruments it is very economical and.

plitude, and-the voltages are consequently re quired to be amplified to` a relatively high` degree before translation into sound of average musically useful intensity. kThe use for this pur- 49 pose of conventional types of amplifying systems employing indirectly heated cathode tubes operated with alternating current heater supply is -likely to result in excessive hum, because of the high degree of amplification following the early stages. The useof amplifying systems in which the cathodes of the tubes employed in early stages are of the filament type and are connected in series and energized from reasonably well filtered rectified current has proven superior. 5g We have foundghowever, that great reduction in the residual hum produced by such systems vmay be effected by suitable modiiications of the customary circuits and methods of adjustment.- l

Accordingly it is an object of ourinvention to provide -means and methods f or reducing' the "assignors to Miessner Inventions, Inc., a cororation of New Jersey application April 9, 1962, serial No. 604,156 9 claims. (c1. 179-171) in series withV each other from a source of recy tiiied current. It is a general object to provide an improved amplifying system of such type. More specific objects of our invention are the provision of means and methods for reducing the residual hum in such an amplifying system in which the input circuits of a plurality of the tubes have common portions; in such an amplifying system in which the input circuit of one or more tubes includes electrostatic apparatus; and in amplifying systems adapted for use in electronic musical instruments of the type above described.

In the detailed description of'our invention,

hereinafter set forth, reference is had to they accompanying drawing, of which Figure 1 is a schematic illustration of ourl invention in one form; and

Figure 2 is a partial such illustration of a modification of the circuits shown in Figure 1.

In Figure 1` is shown diagrammaticallya rectifying and ltering device 119, operable from an alternating current supply and yielding a direct current output having an appreciable residual ripple, or pulsating component. Connected between the output terminals of the rectifier and lter 119 may be the space-discharge paths of tubes5 and 6, in series with a furthervcircuit hereinafter described. Tubes 5 and 6 may be of the so-called power type, operating at relatively high anodevoltages with relatively high space or anode currents and having output or anode circuits connected in the Well understood push-` pull arrangement. Thus transformer '15 is provided, a respective half of its primary being connected in each such anode circuit and its secondary feeding for example'loudspeaker 118. The filaments of tubes 5 ,and 6 may be energized 'in any manner, as by a low voltage alternating current. 1

The further circuit in series with the spacedischarge paths of tubes 5 and 6 may begin at the center-point of a resistance llshunting the filaments of these tubes, and is shown comprising in series arrangement resistance 125; the filament of tube 4 shunted by resistance 14; resistance 124; successively the filaments of tubes 3, 2 and 1 shunted respectively by the resistances 13, 12 and 11 and resistance 1,23. The approximate choice of values of the resistances embraced in this circuit may be outlined as follows:-

Resistance 123 may be of such a value that the total anode current of all the tubes, including 5 and 6, flowing through it will produce a voltage of satisfactory value for grid bias of tube y1 with respect to its filament-preferably the minimum satisfactory bias. Resistance 11 may be of such value as to by-pass around the filament of tube 1 the combined anode current of tubes 2, 3, 4, 5, and

46 in excess of the filament excitation current appropriate for tube 1. Resistances 12, 13 and 14 may be of such values as to by-pass respectively around the filaments of tubes 12, 13 and 14 the ex-1 cess of current owing in the system over the requirements of those respective iilaments, in similar manner as in the case of resistance 11 and tube 1. Resistance 124 may be of such value that the voltage drop in it, added to the voltage drops in the laments of tube 3, 2 and 1' and in resistor 123, will be appropriate for grid bias of tube 4 with respect to its filament, tube 4 being preferably of the semi-power, or medium bias, medium anode current, type. Resistance 125 may be of such value that the voltage drop in it, added to the voltage drops in resistors 123 and 124 and in the filaments of tubes 4, 3, 2 and 1, will be l appropriate for grid bias of tubes 5 and 6 with respect to their filaments.

Plate or anode voltage for tube 4 may be derived from the positive output terminal of rectifier and filter 119, such voltage being reduced in value by resistance 116 and being further filtered by the combination of such `resistance and condenser 117. Plate or anode voltage for tubes 3, 2 and 1 may be derived in turn from the anode voltage supply for tube 4, being reduced in value by resistance 114 and further filtered by the combination of such resistance and condenser 115. The filament and anode voltage supplies for the several tubes having thus been described, consideration may now be given to the general functioning of the amplier and the particular use thereof herein illustrated.

In the figure there are sliown in perspective a plurality of tuned metallic strings 111. These may for example be stretched from hitch-pins over triangular metallic bar or bridge 112 to tuning pins, which it is not here necessary to show. Underneath the strings, and sufiiciently `spaced away therefrom to avoid contact there- With under conditions of maximum vibration thereof, are provided members 101 and 102,/at least the top surface of each of which is electrically conductive. 'I'hus between a point on each string and member 101, a small electrostatic capacity exists. The strings all being electrically in parallel by virtue of their connection to'metallic bar 112, the member 101 as one electrode and the strings as the other form a capacity which is the total of the capacity of the individual strings to member 101. Similarly the capacities between the several strings and member 102 together produce a total capacity between the member 102 as one electrode and the strings as the other. Each of these total capacities may be charged to a relatively high voltage by connection of the strings, through a filter formed by resistor 120 and condenser 122, to the positive output terminal of the rectier and filter 119, the conductive portion of each of the members 101 and 102 being connected through resistances hereinafter mentioned, including 91 and 92 respectively, to the negative output terminal of 119. Resistances 91 and 92 being chosen of very high value, the charge in each of these total capacities will remain relatively constant. If now any string be vibrated in a vertical plane, the capacity between it and each of the members 101 and 102 will be varied in accordance with the frequency and waveform of the point of the string opposite the respective member; the total capacity between all the strings as one electrode and the respective member as the other will be likewise varied, but in reduced degree; and because of the relative constancy of the charge in these capacities, the voltage across each will vary in a similar manner. The strings, being heavily by-passed to the filament system through condenser 122, may be considered as effectively at ground potential from an oscillation stand-point. It may therefore be considered that between ground and each of the members 101 and 102 an A; C. voltage is produced of frequency and waveform corresponding to those of ,the vibrations of the string points opposite the respective member. This action may be considered the counterpart, in translation of the string-point vibration into electric oscillations, of the action of the condenser microphone in translating into similar energy vibrations of its diaphragm.

The conductive portions of the members 101 and 102 are connected to the grids of tubes l and 2 respectively and by these tubes, and the transformers 71 and 72 in their output circuits, A. C. voltages appearing between the members and ground are amplified. By center-tapped potentiometers 81 `,and 82 shunted across the secondaries of the transformers any fraction up to half of the amplified voltages derived fromthe respective two members, in either of two phases, may be combined and applied to the grid of tube 3. By tube 3 and transformer'73 this composite voltage may be amplified and applied across potentiometer or volume control 130, from which any desired fraction of the composite voltage may be applied to the grid of tube 4; and by this tube and transformer 74 it may be further amplified and applied to the grids of tubes 5 and 6, in opposite phase on these two grids for push-pull action, as will be understood.

The function of the plurality of members 101 and 102, of the plurality of tubes 1 and 2 and transformers 71 and 72, and of the potentiometers 81 a'nd 82 is to permit variation of tone quality, or harmonic structure, as is disclosed in the United States Patent' No. 1,906,607, issued May 2, 1933, tc Charles T. Jacobs. Such harmonic structure control arises from the diiference in harmonic composition, or waveform, of the vibrations of the mean string point opposite the two members 101 and 102, the consequent dilerence in harmonic composition of the A. C. voltages applied to the grids of tubes 1 and 42, and the infinite variety of amplitude relationships and the dual phase relationships in which these voltages may be combined or algebraically added, after amplification, by various adjustments of the potentiometers 81 and 82. The number of such translatingsystems need not of course be limited to two, such number being only illustrative herein.

In an amplier system of the character herein shown and described, each tube must be so supplied with filament and anode current and grid bias voltage that its actual anode current is substantially free of ripple, or residual pulsation originating in the rectifier and lter 119. In the system shown it will be seen that progressive lor group of tubes is filtered in a degree varying with the degree of amplification following the tube. No such progressive filtration is readily available for lament current, however, this supply for each of the tubes 1, 2, 3 and 4 being filtered in general only as well as the anode crrrent 1 employed. It will be understood, of course, that y of tubes 5 and s. since these last mentioned kment, to disconnect the electrostatic portions of the grid circuits from tubes l and 2. To this end We have shown switches 131 and 132 respectively; it will rst be assumed that these switches are open as shown.

For any tube whose filament energization contains a ripple component there is an optimum point between the extremities of the filament to which anode and input circuits should be connected in order effectively to avoid the introduction of some of the filament ripple into such circuits. This point may of course be effectively reached without actual connection within the tube, by connection to a point of 'similar potential on a resistance shunting the tube. It is obvious, however, that no one point or potential in a series filament system can be such optimum for more than one tube; and that when a single anode 'supply lter is employed for several tubes, as resistance -114 and condenser 115 for tubes 1, 2 and 3, the anode voltage applied to .these tubes issubstantiall'y free from rippple with respect only to a single point in the filament systeme g., the point of, connection of the filter condenser 115 thereto. This point, lying outside the filaments of each of the tubes 1 2 and 3, is not the optimum point for any one; and consequently a ripple voltage will be introduced in the anode circuit of each of these tubes, determined for each tube in phase and amplitude by the relative positions and mean potentials of the optimum point for that tube and the-point of connection of condenser 4115 to the filament system. It will furtherbe appreciated that in the case of a single tube; if the anode supply therefor beby-passed to a point Whose potential is not that of such optimum point-e. g., to any point not intermediate the extremities of the filament in potentiala ripple voltage will be introduced into the anode circuit, and that our invention is useful in connection with such a single tube case as well as with each tube of a plurality. y y

To make the anode current free of ripple or uctuation despite the existence of ripple voltage in the anode supply, we introduce into the grid circuit a ripple voltage from the filament system--the source of anode circuit ripple voltageand adjust the amplitude, and at the same time Athe phase between in-and out-of-phase conditions, until vthe anode current is free of ripple. The ability to do this rests upon the familiar Iprinciple that if `a voltage be introduced into the -grid circuit of a tube; such voltage times `the amplification factor (mu) of the tube will appear in the plate circuit.

Thus in the -figure appears a resistance 113 shunted around the entire filament system of tubes 1, 2, 3 and 4, such system being considered to include resistors 123, 124 and 125. To avoid shunting an appreciable amount of current by the filament system, resistance 113 should?. be of relatively high value; if the shunting eii'ect be taken into consideration in the choice of values 0f resistors 123, 11, 12, 13, 124, 14 and 125, hOW- ever, lower values for 113 'may equally well be.

the optimum point of grid and plate .circuit i connections for each tube, or more exactly the potential of each such optimum point, appears on resistance 113. By connecting; A. C. carrying paths from the grid circuits of thetubes to suitable points on the resistance'll not coincident with the respective optimum points, ripple voltages may be introduced into the grid circuits of the respective tubes of correct magnitude and phase to oppose anode supply voltage variations.

In Figure l it will be seen that there is a'D. C. y

voltage drop along resistance-113. For proper establishment of the D. C. grid bias voltage for each of the tubes it is therefore necessary to provide a D. C. carrying path, in addition to theabove-mentioned A. C. carrying path, from eachv grid circuit to the filament circuit. Thus the lower potential end of resistance 91 is connected through resistance 21 to the negative end ofv the filament system, as well as being connected through condenser 31 `to slider 41 on resistance 113. D. C. cannot pass through condenser 31 and is therefore confined to the path through resistance 2l. On the other hand, provided the reactance of the condenser 31 at ripple frequencies be negligible compared to the value of resistance 21, A. C. potentials appearing at the slider 41 will afllpearwvith negligible alteration of phase or am; plitude at the lower potential end of resistance'91 and thus be applied to the grid of tube l. l

It is sometimes convenient to make similar the D. C. potentials of the two members 101 and 102.

Since the filaments of tubes 1 and 2 are n'ot-at the same potential this results in a slight-grid bias voltage difference between tubes l and 2, whichwill not usuallyl be found objectionable, however.`

Thus the D. C. path from the grid circuit of tube4 2 is shown throughl resistance 22 also to the negative end of the filament system. The A. C. path from this grid circuit isthrough condenser 32 to slider 42 on resistance 113.

The D. C. path 'fromthe ygrid circuit of tube 3 is lshown through resistance 23 to the negative terminal of filament of" tube 1, and the A. C. path through condenser v33 to slider 43 on resistance 113. The D. C. path from the grid circuit of tube 4, which tube has an effectively separate anode current supplyy,v is shown through resistance 24 to the vnegative end of the filament system, and an A. C. path fromy such grid circuit through condenser 34 to slider 44 on resistance 113. By way of example of the values of components which we have employed, resistors 21, 22, 23 and 24 may be of 1 megohm each and 'condensers-Bl, 32, 33 and 34 of .1 microfarad each, resistances or grid leaks 91 and 92 being of 21/2 megohms or greater. f

In adjusting the sliders 41, 42, 43 and 44 it is convenient first to adjust the position of slider 44 for minimum hum in loudspeaker 118 with the slider of volume control 73 temporarily in the down-most position; next likewise to adjust the position of slider 43, with the sliders of potentiometers 81 and 82 each at the respective midpoint tap; next to adjust the position of slider 42, with the slider of potentiometer 82'at one extremity and that of potentiometer 81 at the mid-point tap of their respective potentiometers;

and finally to adjust the position of slider 41, with the slider of potentiometer 81 at one extremity and that of potentiometer 82 at the mid-point tap. Thisorder of adjustment cuts out of -cir- During the preceding adjustments it has been assumed that switches 131 and 132 were' open. The system being otherwise so adjusted as to produce negligible hum in loudspeaker 118, it may be found upon closing of these switches that hum will be introduced. This may be accounted for as follows:-

Between the optimum point of tube 1 and the negative end of the filament system a riple voltage exists. Because of the negligible reactance of filter condenser 122 this ripple voltage also appears between such optimum point and the strings; and it is applied, through the total capacity between the strings and member 101, to the grid of tube l. A similar ripple voltage is likewise applied to the grid of tube 42. If, however, we connect to the grid of tubev 1 a capacity, say equal to the total capacity between the strings and member 10,1, and connect the other side of this capacity to a point in the filament system whose ripple potential difference from the optimum point of tube 1 is equal, but of opposite phase, to that of the negative end of the filament system, the grid of tube 1 will be subjected to equal and opposite ripple voltages and will consequently respond to neither. Thus we have shown a variable capacity 5l connected between the grid of tube 1 and a slider 61 movable on resistance 113; and for tube 2 there may be connected a capacity 52 between the grid of that tube and slider 62 on resistance 113.

Condenser 51 and slider 61, provided for tube 1, may be adjusted together, the slider of potenl tiometer 82 being positioned during the adjustment at the mid-point and that of potentiometer 81 at one extremity of those respective potentiometers for elimination of any effects from tube 2 and its circuits. The value of condenser 51 need not necessarily be equal to that of the capacity between the strings and member 101, since a particular position of the slider 61 may be found, for each of a large number of values of the condenser 51, which will reduce the hum from the presently discussed source to a negligible value. ln general it is preferable to employ a very low value of capacity 51 in order to avoid appreciable by-passing effect on signal voltages, with which low value will be found appropriate a setting of slider 61 well toward the right-shown end of resistance 113. plicable to the adjustment of condenser 52 and slider 62, the respective tubes 1 and 2 and respectively associated circuit elements being interchanged in reading the remarks.

It will be understood, of course that resistance 113 may be replaced by a plurality of resistance elements each shunting only a portion of the filament system; some of them, for example, replacing some of the resistors 123, 11, 12, 13, 124 and 125 if desired. The single resistance 113 with the plurality of sliders or other adjustable contacts, however, not'only most readily illustrates the principles of the invention, but also very conveniently provides an unbroken range of adjustment for each slider.

It will be noticed in Figure -1 that the D. C. path from the grid circuit of each of the tubes 1, 2 and 4 is connected'to the negative end of the filament system. When several of the tubes may be employed with such common point source of D. C. grid bias voltage, it may be found convenient and economical to employ the alternative embodiment of our invention. shown inFigure 2, which is a portion of Figure 1 redrawn to illustrate thev modification. Herein D. C. is kept out of resist-I ance 113 by condenser 126, the mean D. C. po-

The same remarks are apf incase.,

tential of the entire resistance 113 thus becoming that of the negative end of the filament system. The condenser 126 should have a reactance at ripple frequencies negligible compared to the value of resistance 113, in order that the ripple current flowing in resistance 113 shall be of substantially the same phase as that fiowing through the filament system proper. Since any point on resistance 113 is now of the correct D. C. potential for bias of the tubes 1, 2 and 4, the division of the grid-to-filament circuit connections of these tubes inta D. C. and A. C. paths may be eliminated, and the resistors 2l, 22 and 24 and condensers 31, 32 and 34 omitted, as seen in Figure 2.

It will be understood that various modifications of the circuits shown, and applications of the princip'es disclosed to various other circuits and uses, may be made without departing from the spirit or scope of our invention, as in the appended claims defined. A

We claim:-

l. In an amplifying system, the combination of an electron tube having at least a lament, a grid and an anode; a circuit including said lament and a source of pulsating direct current; a grid circuit for said tube, including electrostatic apparatus connected between said grid and a point in said filament circuit; and an adjustable capacity connected between said grid and a point in said filament circuit whose pulsating potential with respect to a point within said filament is of opposite phase to that of said first mentioned point.

2. In an amplifying system, the combination of an electron tube having at least a filament, a grid and an anode; a circuit including said filament and a source of pulsating direct current; a grid circuit for said tube, including electrostatic apparatus connected between said gird and a point in said filament circuit; and means for opposing pulsating potentials applied to said grid through said electrostatic apparatus, said means comprising a capacity connected betweensaid vapplied to said grid through said electrostatic apparatus, said means comprising a source of pulsating potentials with respect to said cathode of opposite phase to those first mentioned, and a capacity connecting said source to said grid.

1. In an amplifying system, the combination of an electron tube having at least a filament, a grid and an anode; a circuit including said filament and a source of pu`sating direct current; an anode circuit for said tube effectively including a portion of said filament circuit; a grid circuit for said tube including electrostatic apparatus apparatus connected between said grid and a point in said filament circuit; means for biasing said grid; low reactance means for introducing into said grid circuit a pulsating potential from said filament circuit to oppose fluctuations of.

anode current arising in said anode circuit; and high reactance means for introducing into said grid circuit a, pulsating potential from said filain a common` circuit including a source of pulsating direct current; anode circuits for said tubes respectively, the several said anode circuits 'including a common alternating current carrying path which terminates at a single point in said lament circuit; grid circuits for said tubes respectively, high impedance means connected from saidI filament circuit to said grid circuits for intro-14 ducing into said grid circuits biasing potentials for said grids; and individual low impedance means connected from said lament circuit to one or more of the several said grid circuits for introducing into said grid circuits pulsating pctentials to oppose pulsating potentials introduced into the corresponding said anode circuits by virtue of said common return path.

6. In an amplifying system, the combination of a plurality of electron tubes each having at least a lament, a grid andan anode, each said tube having anvinput circuit and each said lament being connected in a common circuit in series with a source of `pulsating direct current; a circuit comprising a resistance element and a condenser serially connected therewith, said lastl mentioned circuit being shunted across at least a portion of said illament circuit; movable contacts on said resistance element; and a connection from each said input circuit' to a respective said movable contact.

7. In an amplifying system, the combination of an electron tube having a lament, a grid and an anode; a circuit including in series said filament, a source of pulsating direct current, and resistance; a circuit comprising a resistance element and a condenser serially connected therewith, said last mentioned circuit being shunted across at least a portion of said filament circuit; a movable contact on said resistance element; and an input circuittor said tube connected from said grid to said movable contact.

8. In an amplifying system, the combination of at least one electron tube operating with a relatively high space current; a plurality of electron tubes operating with filament currents less than the space current of said first mentioned tubes; a circuit in series with the negative end of the space discharge paths of said rst mentioned tubes, said circuit including in series the filaments of said second mentioned tubes; means for supplying pulsating direct currentto said space discharge paths and said circuit in series; means for applying at least a portion of the D. C. voltage drop in said circuit to the grids of said first mentioned tubes to bias the same; means for applying portions of the D. C. voltage drop in said circuit to the grids of said second mentioned tubes to bias the same; and means forapplying pulsating potentials from said circuit to the grids of said second mentioned tubes.

9. In an amplifying system adapted to be used with a musical instrument of the type wherein the vibrations of a plurality of tuned bodies are electrostatically translated into electric oscillations by adjacent stationary conductive members having a D. C. potential diierence from said tuned bodies, the combination of an electron tube for each of saidconductive members; a circuit including the llments of said tubes in series with each other and with a` source of pulsating direct current; a path from the grid of'each said tube to a respective said conductive member; a path from said tuned bodies to a point in said filament circuit; means for biasing each of said grids; and

`individual means for applying to said grids pulsating vpotentials from .said filament circuitA to oppose like potentials thereto applied through the capacities between said tuned bodies and said conductive members.

BENJAMIN F. MIESSNER. CHARLES T. JACOBS. 

